Why You Shouldn't Let Your Dog Pee on Trees

Anyone who's ever owned dogs knows that the lovable creatures are potent forces of destruction. From vomiting squirrel parts on the sofa, to inhaling the girlfriend's laundry-basket panties, to arranging a trashcan's contents over the floor with the gusto of an FBI forensics team, some days it seems as if canines were bred solely to generate hilarious barroom anecdotes.

But might dogs be engaged in a more clandestine, less-funny campaign of attrition, one that is harming the very health of our urban landscape?

Carrie Maria certainly thinks so. As owner of one of Philadelphia's biggest dog-walking services, the "Monster Minders," Maria knows hounds – and she suspects they are destroying her city's plant life.

A couple of years ago, Maria took a walk around her home in Bella Vista armed with a camera. Following a sidewalk route frequented by many bowwows, she was able to amass a photographic collection of disfigured trees in a matter of minutes. First, there was this mangy-looking specimen:

On this trunk, the bark was peeling off as if somebody had scorched it with hot lye:

Here's a tree bearing a mysterious white ring that looks bleached:

Maria isn't a total vegetation naif, having graduated from the Pennsylvania Horticultural Society's Tree Tenders program. After examining the evidence around her, she concluded that the best explanation for the mortally wounded trees was that dog owners let their pets repeatedly urinate on them.

"All over the city you can actually see evidence of burning on the trees. It's super easy: Just look down, and at the lower foot or two feet of the tree you can see fissures and cracking," she says. "I first thought, well, maybe it's just the salt" that municipal trucks spread during winter. "Then you notice that the trees with cages around them don't have these marks."

Urban trees are dealt a bad hand. They are deprived of adequate and good-quality soil, exposed to foreign chemicals, smacked by car doors and sometimes run over by construction equipment. In this harsh environment, a number of irritations can add up and drag a plant into a seriously bad state. Could dog urine be one of the things pushing our trees toward doom?

The Origins of Pee Theory

Whether pee hurts trees is a question that's attracted virtually no research attention since its earliest mention in the academic literature (earliest that I can find, anyway), "Why shade trees die along city streets," a presentation given in 1959 by Pascal Pirone at the International Shade Tree Conference. A plant pathologist at the New York Botanical Garden, Pirone was the first person to sound the klaxons on what he called “dog canker.”

This affliction typically manifested in the lower 2 feet of the trunk, he wrote, and could kill trees up to 6 inches in diameter. He said that a dog-heavy community might try locking a "metal collar" around a pee tree, but added it would be almost useless to do so, because "the dog's urine will still seep into the soil and root area to cause severe damage to the roots and premature death of the tree."

Pirone was not available to be interviewed about the plant-bane of dog piss. He escaped his earthly vessel in 2003, leaving behind the intriguing clue that it had to do with "large amounts of potassium." So I called a bunch of arborists and parks officials and asked them about the possible wounding effect of dog pee on vegetation.

The Smithsonian Institution's horticulture department was a notable skeptic of the "number one" theory. One staffer told me that the Philadelphia photo array “shows pictures of trunk damage on various trees, but this type of damage can be from many causes, including mechanical damage [i.e. mowers, car doors, pedestrians], southwest injury, disease [cankers], and insects [borers]."

Still, the general consensus does appear to be that, yes, an army of tail-waggers can be detrimental to a tree's health, particularly if it's young or has thin bark. Knowledgeable sources who believe Dog-Wee Death is real say it goes down in several different ways, as I'll explain shortly. But there's no firm agreement on which pee-mechanism is the most crippling.

"We deal with it in the sense that I imagine trees get added stress or maybe anxiety" from dogs, says John Thomas, associate director at Washington, D.C.'s Urban Forestry Administration. "I don't know how much dog urine you need to kill a tree. But there's definitely something there.... Somebody could definitely get a masters or Ph.D. out of studying it."

This deficit of information has left dog owners and tree lovers in the dark when it comes to the danger of yellow emissions. The result can be uncomfortable encounters between strangers passing on the street. Take this man's account of butting heads with his Park Slope neighbors after they spotted his pup unleashing "3 drops of pee" in a Polhemus Place tree box:

They told me to "stay off their block" and one of them took my picture, saying something about the block organization and everyone knowing who I am. Then one of them did the "shooing" gesture. I told them I thought something was wrong with both of them, and walked on.

So much about this chapped my lowly 7th Ave ass: The entitlement of these Polhemus residents, and THEIR trees, which are planted by the city. The picture, which made me feel like a playground perv. And the accusation that I was a bad dog owner AND a bad person, because urine was killing the trees.

And that wasn't nearly as bad as when another Park Sloper allowed his "big goofy friendly" pooch to dose a neighbor's shrub. That dog lover was allegedly informed, "Next time I see you, I’m going to fucking kill your dog."

A Dog's Love Affair With Trees

Why do dogs pee on trees? Or really on any vertical structure within hose range?

For the longest time, researchers thought it had to do with marking territory, kind of like how a gang member might tag a wall to scare off rivals. But recent investigations suggest that the urine squirts are much more complicated, and instead constitute a rich form of social interaction.

Psychologist Alexandra Horowitz, who works at Barnard College's Dog Cognition Lab, has studied the phenomenon long and hard. In her 2009 book, Inside of a Dog: What Dogs See, Smell, and Know, she argues that canines whiz on objects to give each other information on sexual readiness, social confidence and the crucial detail of whether they're a dog-about-town (meaning, how often they pass a location). "In this way," Horowitz writes, "the invisible pile of scents on the hydrant becomes a community bulletin board, with old, deteriorating announcements and requests peeking out from underneath more recent posts."

Last year, Wisconsin researchers who were monitoring dogs at a park called Muttland Meadows, and also at an experimental "urine course," concluded that the peeing had much to do about status. How'd they figure that? Well, they noted which dogs enjoyed the most popular regard by measuring the angle of their tails – the higher the angle, the more important the dog – and then correlated that with how often they "overmarked," or peed over another dog's urine. High-status dogs turned out to be the ones that overmarked the most.

Through that lens, what we view as a dog just ejecting its bladder contents everywhere is actually a crafty and ever-shifting game of one-upsmanship. Or a booty call. Whatever the case, keeping up the dialogue requires a whole lot of urine in play, and that's where the trees start to suffer.

The problem begins when dogs decide that one trunk is the hot place to moisten. This swarming can happen outside a pet-friendly apartment building, in a dog park, or in a neighborhood with a healthy dog population and tree boxes on the sidewalk. Carrie Maria has seen firsthand these canine pee orgies. "I know that there are trees in our neighborhood that dogs just gravitate toward," she says. "I don't know if it's one dog peeing on a tree, and then the rest of the dogs are like, YES! This is the tree we're all going to pee on."

When dogs single out a tree for a pee party, the plant sustains impairments in at least three areas. These are:

The bark

Regularly slathering a tree's skin with urine can cause "ammonium toxicity," says Nina Bassuk, program leader at Cornell University's Urban Horticulture Institute. "Ammonium is in all mammals' pee. It is a nitrogen source so a little bit is OK, but a lot is toxic.... It's a question of a little too much of a good thing."

Under its scaly armor, a tree has a layer of tissue called the cambium that makes it grow in diameter. The chemicals in urine can soak through exterior bark and damage this vital substance, explains Bassuk, either destroying the tree or impairing its growth. "It's like cutting off part of the circulatory system," she says. And a tree with a dysfunctional bark is easy prey to burrowing insects and oozing diseases like "bacterial wetwood."

The roots

A spokesperson for the New York City Department of Parks & Recreation e-mailed this statement when put to the dog question:

When urine is added to a tree pit, the extra salt can create a crust on the soil, which makes it almost impenetrable to water. Salt also draws out water from tree roots, further compounding water loss and simulating the effects of drought.... These problems are exacerbated because dog urine attracts more dogs to do the same. Tree pits are very limited in water, air, soil, and nutrient availability. The soil is also very compacted, which further intensifies these limitations and damages. Therefore, it’s important to limit animal waste in the pit to help keep the tree as healthy as possible so that it can fight off pests and diseases and grow to its full potential.

Sam Bishop, education director for Trees New York, says that on a dense city block where everybody's dog ends up using the same tree, the dirt can become highly polluted with salt. His organization has planted thousands of trees around New York City and now feels forced to defend them with signs reading, "Curb Your Dog."

Bishop says it is especially frustrating when he runs into New Yorkers who believe that their furchildren are actually doing the tree a favor. "I've heard people jokingly refer to it as 'watering the tree,' and some people think it's good for the plant and is fertilizing it," he says. "But no, no, it is not good for the plant."

The topsoil

Dogs tromping around in a tree box or a public park every day can make the ground as hard as "a base to pour a sidewalk on," says D.C.'s John Thomas. The constant pounding of thousands of paws squishes macrochambers of soil-locked air down into microchambers, destroying the earth's natural architecture and starving upper roots of oxygen. The surface becomes nearly impenetrable, so that when storms come the tree stays parched of rain.

"We see it in certain parks, like the one up by Mount Pleasant in the dog area," says Thomas. "Those trees were not doing so great before they started. They're doing even worse now because of compaction and feces and urine."

The D.C. government has reached out to pet owners to mitigate dog-based deforestation – for instance, working with communities to put up signs that might help protect a green space. Thomas says the results thus far have not been encouraging.

"It's hard, because dog owners are pretty avid, or pretty rabid, depending on what group you're dealing with," he says. "I love dogs and dog owners, but it's just a hard group to penetrate. They are kids to them, and they can get very defensive. We've had very little luck when we've gone into this kind of situation."

If dogs are truly our surrogate children, why do we allow them to behave badly? Certainly most parents wouldn't let a toddler rip strips of bark from a city block's precious few trees.

No doubt much of it has to do with ignorance. That might change soon, as more and more pet owners flock to cities and drive up the costs of plant damage. In fact, you can already see that happening, according to Keith Pitchford.

Pitchford is an arborist who works with a citizen's group to plant new trees in D.C.'s Georgetown neighborhood. About five years ago the trees went into the ground unprotected. Now they go in barricaded with an attractive but formidable wrought-iron fence. These barriers are designed to ward off car-door punches, stampeding pedestrians and – you guessed it – steaming mutt juice.

"The final straw was really the dogs," says Pitchford, who adds that the metal-enhanced trees cost about $750 to erect, $400 more than the old, unguarded ones. "We used to plant 50 to 75 trees a year. Now we're putting fewer in, because we need more money for fencing." (Little does Georgetown know that ironwork is alsosusceptible to pee corrosion.)

Up in Manhattan, in the vast urinefields of Battery Park City, conservators have lodged complaints against what they deem reckless dog owners. "The health of trees at park entrances is seriously undermined by the carelessness of dog walkers," they gripe. The burden of cleaning off stained granite surfaces and replacing pee-mottled plants dings the park's tenders "tens of thousands of dollars annually."

In some cases, people seem to either not believe, or not care, that their doggies might be tearing holes in the urban scenery. After all, everyone knows why cities need pooper-scooper laws – visit Naples if you don't understand why leaving big turds on the sidewalk is a hassle – but what harm could a puddle of pee possibly cause?

This reasoning was on display in a dialogue among users of a D.C.-area Internet forum. After one homeowner politely asked her neighbors not to let their animals pee in her yard – oh yeah, dog urine kills grass, too – "nthehvns" came out of the woodwork to offer this presumptuous rebuttal: "Are dogs not allowed to pee while they are being walked? My dog pees in my yard and others while he is being walked. I'm not sure how I could even stop this."

Believe it or not, there is a way to stop it. That's not to try to biohack your dog to alter its urine chemistry, despite all the folksy prescriptions for tomato-juice or baking-soda cocktails. It's to curb the animal, which means train it to unload in the gutter. That's where you're supposed to do it anyway, according to laws in many large cities like New York.

"We all have to share public spaces, and as dog owners if we want to get respect from our neighbors, we have to give them respect," says Philadelphia's Maria, who recently put out a sign requesting that passersby allow her, and not dogs, to water the tree in front of her home. "It's not anti-dog. It's just pro-tree."